1. Field of the Invention
The present invention relates to an organic electrolytic solution and a lithium battery employing the same. More particularly, the invention relates to an organic electrolytic solution which can effectively suppress swelling of a battery, and a lithium battery employing the same.
2. Description of the Related Art
As portable electronic devices such as a video cameras, mobile phones or notebook type personal computers become lightweight and highly functional, extensive research into batteries used as power sources for driving the portable electronic devices is underway. In particular, rechargeable lithium secondary batteries have gained most attention and are vigorously studied because they have approximately three times a higher energy density per weight and are rapidly rechargeable, compared with conventional batteries such as Pb storage batteries, Ni—Cd batteries, Ni—H batteries or Ni—Zn batteries.
When a liquid electrolyte is used for a lithium secondary battery, various problems are encountered in view of safety, such as risk of fire due to leakage, damage to a battery due to evaporation. To avoid such problems, attempts to use a solid electrolyte, instead of a liquid electrolyte, have been made.
In general, since a solid electrolyte is free from risk of leakage of an electrolytic solution and easily prepared, compared to a liquid electrolyte, it has attracted much attention and extensive research is underway. Specifically, polymeric solid electrolytes are vigorously studied. Known polymeric solid electrolytes are classified into a solid type in which no organic electrolytic solution is contained at all, and a gel type in which an organic electrolytic solution is contained.
A lithium battery driven at a high operating voltage cannot employ a conventional aqueous electrolytic solution because lithium of an anode violently reacts with the aqueous solvent of the electrolytic solution. Thus, an organic electrolytic solution having a lithium salt dissolved in a nonaqueous organic solvent is used as an electrolytic solution for a lithium battery. As the organic solvent, a low-viscosity organic solvent having high ionic conductivity and a high dielectric constant is preferably used. However, a nonaqueous organic solvent satisfying these requirements does not exist in a single form. Thus, a mixed solvent of a highly dielectric organic solvent and a low viscosity organic solvent, can be used.
U.S. Pat. Nos. 6,114,070 and 6,048,637 disclose a method of increasing the ionic conductivity of an organic solvent using a mixed solvent of chain carbonate and cyclic carbonate, that is, a mixture of dimethylcarbonate or diethylcarbonate and ethylenecarbonate or propylene carbonate.
However, the mixed solvent can only be used at 120° C. or less but cannot used at a temperature higher than 120° C. because gas is generated due to vapor press, resulting in swelling of a battery.
U.S. Pat. Nos. 5,352,548, 5,712,059 and 5,714,281 disclose an electrolytic solution comprising an organic solvent having at least 20% vinylene carbonate (VC). However, since VC has a smaller dielectric constant than ethylene carbonate, propylene carbonate or γ-butyrolactone, charging/discharging performance and high-rate performance of a battery considerably deteriorate when it is used as a main solvent.
U.S. Pat. No. 5,626,981 discloses a method of forming Surface Electrolyte Interphase (SEI) on the surface of an anode by adding VC to an electrolytic solution at an initial charging/discharging cycle. Japanese Patent Laid-Open Publication No. 2002-33127 discloses uses of an additive for an electrolytic solution having electrochemically stability. Also, Japanese Patent Laid-Open Publication No. 2001-217001 describes use of an additive for a nonaqueous electrolytic solution of a secondary battery providing inflammability, good low-temperature performance and reduced interface resistance by using a phosphagen derivative.
Japanese Patent Laid-Open Publication No. 2001-176548 describes that the irreversible capacity of a lithium ion cell is minimized during the first cycle and cycle capacity of the cell is maintained at low temperature by employing a sulfuric ester additive.
However, when such batteries are stored at high temperature, the cell thickness exceeds an allowable limit, resulting in deformation of a battery pack, making use of the battery impossible.
Japanese Patent Laid-Open Publication No. 1999-273725 discloses improvement of a battery by use of 2(5H)-furanone. According to this patent, in a battery system using LiCoO2 or LiMn2O4 as a cathode active material and natural graphite as an anode active material, when 0.1˜5 wt % of 2(5H)-furanone is used, the charging/discharging efficiency of the battery is improved with repetition of 50 cycles. However, use of natural graphite as an anode active material lowers the overall efficiency of the battery.